In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization of the MR element, which in turn causes a change in resistance of the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the GMR sensor varies as a function of the spin-dependent transmission of the conduction electrons between ferromagnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the ferromagnetic and non-magnetic layers and within the ferromagnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni—Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer (reference layer), has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe—Mn) layer. The pinning field generated by the antiferromagnetic layer should be greater than demagnetizing fields (about 200 Oe) at the operating temperature of the SV sensor (about 120° C.) to ensure that the magnetization direction of the pinned layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field).
The discovery of GMR has spurred a body of investigations on magnetotransport properties in different sample systems. One such area of investigation has led to the development of ballistic magnetoresistance (BMR) structures that may be useful in applications relating to disk drive systems, e.g., spin valve devices. At room temperature and low applied fields (<100 Oe) very large BMR values, larger than 300% over GMR values, can be achieved in metallic nanocontacts of a few atoms size. More detail on the theory behind BMR is provided in articles entitled “Ballistic magnetoresistance in different nanocontact configurations: a basis for future magnetoresistance sensors”, N. Garcia et al., Journal of Magnetism and Magnetic Materials 240 (2002), pp. 92-99; and “From ballistic to non-ballistic magnetoresistance in nanocontacts: theory and experiments”, Y.-W Zhao et al, Journal of Magnetism and Magnetic Materials 223 (2001), pp. 169-174. These articles and all documents referenced therein are herein incorporated by reference.
What is needed is a practical application of BMR into a magnetic sensor that can be used with disk drive systems.